Open relationships are a lot of work. And both partners need to be very honest with themselves and aware of what their hot buttons are.
I've been in a handful over the years, and the demise of each one was jealousy. At first, everything was great and fun, but as soon as one of the primary partners feels threatened by one of the playmates (too young, too pretty, too buff, too smart...) problems happen. Resentments build. "You can, but I can't?" Endings are often very dramatic.
I think the most stable way to have openness in a relationship is to always play together, and to give veto power to both partners. That works as well as anything else I've tried.